The present invention relates to a coaxial cable, and more particularly to an improved low-loss coaxial cable having enhanced bending, handling and electrical properties.
The coaxial cables commonly used today for transmission of RF signals, such as cable television signals and cellular telephone broadcast signals, for example, include a core containing an inner conductor, a metallic sheath surrounding the core and serving as an outer conductor, and in some instances a protective jacket which surrounds the metallic sheath. A dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In many known coaxial cable constructions, an expanded foam dielectric surrounds the inner conductor and fills the space between the inner conductor and the surrounding metallic sheath.
The design of coaxial cables has traditionally been a balance between the electrical properties (e.g., high signal propagation, low attenuation) and the mechanical or bending properties of the cable. For example, in some coaxial cable constructions, air and plastic spacers are used between the inner conductor and the outer conductor to reduce attenuation and increase signal propagation of the cable. Nevertheless, the plastic spacers that are placed between the inner and outer conductors do not provide much support in bending for the outer conductor and thus the outer conductor is subject to buckling, flattening or collapsing of the cable during bending which can render the cable unusable. One alternative has been to use foam dielectrics between the inner and outer conductors as described above. However, although the bending properties are improved, the rate at which the signals are propagated is typically reduced.
For example, EP 504 776 describes a coaxial cable comprising a polytetrafluoroethylene (PTFE) rod that surrounds a copper wire further surrounded by a conductive copper tape that forms the inner conductor. The conductive copper tape is applied by wrapping a tape helically around the supporting rod, by vapor deposition in a vacuum, by cathode sputtering, or chemically. An intermediate dielectric formed of expanded PTFE surrounds the conductive copper tape and is further surrounded by an outer conductor and an outer insulator. The outside diameter of the cable is 3.58 mm based on the diameter of the outer conductor.
One recent advance in the coaxial cable industry for RF cables has been the construction of larger diameter cables. Large diameter cables generally possess a greater average power rating and reduced attenuation over smaller diameter cables. Unfortunately, however, because these cables have large diameters, they are typically not as flexible as their smaller diameter counterparts. As a result, there is a greater level of difficulty in installing these cables. For this reason, large diameter cables have been designed with corrugated sheaths for greater flexibility.
Another problem with the large diameter cables has been that the cost of the large diameter solid inner conductors generally used in these cables is rather expensive because of the large amount of conductive material used. In consideration of this problem, one alternative in the design of conventional large diameter cables has been the use of corrugated metal tubing as the inner conductor. The corrugated metal tubing reduces the expense of the inner conductor and along with the corrugated outer conductor improves the bending properties of the cable. Nevertheless, the metal tubing is subject to the same problems in bending as the outer metallic sheaths typically used in the cables. Specifically, the metal tubing has the tendency to buckle, flatten or collapse during bending of the cable thus rendering the cable unusable. Furthermore, although the coat of the corrugated inner conductive tubing is reduced over solid inner conductors these corrugated inner conductive tubes are still rather expensive. Additionally, the corrugated inner and outer conductors typically cause attenuation and reflection (return loss) of the RF signals and can produce problems during connectorization of the cable.
The present invention provides a coaxial cable having excellent electrical properties, particularly for the transmission of RF signals. In addition, the present invention provides a coaxial cable which has outstanding flexibility and bending properties even for large diameter cables and which avoids buckling, flattening or collapsing in bending. he coaxial cable of the invention is easily connectorized and has good water blocking properties to prevent the flow of water through the coaxial cable. Furthermore, the present invention provides a coaxial cable and a method of making same at low cost.
These and other features are achieved in accordance with the present invention by providing a flexible coaxial cable having a cable core comprising a cylindrical plastic rod, an inner conductor surrounding the plastic rod, and a foam polymer dielectric layer surrounding the inner conductor. A tubular metallic sheath closely surrounds the cable core to provide an outer conductor for the cable. Additionally, the cable can include a protective polymer jacket which surrounds the sheath and can be adhesively bonded thereto. The cylindrical plastic rod comprises a solid or foam plastic material which supports the inner conductor in bending and can be adhesively bonded to the inner conductor. The plastic rod can also be supported by a central structural member to facilitate formation of the plastic rod. The coaxial cables of the invention have been particularly useful for large diameter cables, i.e., having outer metallic sheath diameters of more than 1.0 inches (2.5 cm), but can also be used with smaller diameter cables.
The present invention also comprises a method of making coaxial cables. In the method embodiment of the invention, a cylindrical plastic rod is advanced along a predetermined path of travel and an inner conductor is directed onto the plastic rod and encircles the plastic rod. Preferably, the inner conductor is formed such that it loosely encircles the plastic rod and is then sunk onto the foam plastic rod. In addition, the inner conductor is typically adhesively bonded to the plastic rod. A foamable polymer composition is extruded onto the inner conductor to form a cable core. A tubular metallic sheath is then formed onto the cable core and encircles the cable core. A protective polymer jacket can also be formed surrounding the sheath and can be adhesively bonded to the sheath. The plastic rod is preferably formed by extruding a polymer composition onto a central structural member. The inner conductor can then be formed by advancing a metal strip and longitudinally welding abutting portions of the metal strip around the plastic rod to form an inner conductive tube or the metal strip can be overlapped around the plastic rod.
These and other features of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes both the preferred and alternative embodiments of the invention.